Rubber
No seriously a cheap waterproof rain slicker or pair of rubber boots will bribe the average person very easily, waterproof materials were nonexistent. It is also really easy to demonstrate and you can buy them at a dollar store. The best they had at the time were merely water resistant materials which tended to stain since they were soaked in oil. Plus there is no risk of accused of black magic like you might with medicine or any other active chemical. Plus it will rot away in a few decades so no evidence, as long as you don't pass out hundreds of them.
Nails
Nails were valuable enough to be major market item becasue they were time consuming to make. But at the same time, they were common enough that anyone would recognize and want them. Small furniture nails were even more valuable. A box of nails both valuable AND easy to fraction out (20 nails for a piece of whatever I smell cooking), and would not draw that much attention, since they did exist. you can buy square/box nails at any large hardware store, The process is different but the finished project will be identical.
Salt is just easy its desirable and easy to come by, sea salt is considered more desirable, spices are more tricky. You could easily be accused of selling poison or being a smuggler or get your friends accused of the same.
Cotton Go buy a few bolts of cotton cloth from Walmart. You can trade each one for a month of food and shelter without much problem. Go with tan or off-white to reduce suspicion. Remember: you don't want to give out too much wealth because it just makes those people a target. Cotton was known at the time, but it was traded from India at a high cost. Just buy "all natural" or unbleached cotton.
Wooden dice Easy to carry, dice were common, boredom is universal and you can buy them at any craft store cheap.
Sewing needles Light, easy to carry, you can buy them at Walmart by the hundreds for pocket change and everyone in the medieval society will see their value. Go with big over small however, the cloth of the time was rough spun.
Combs Even metal combs are cheap, pretty, and combs are universal so that anyone with hair will see their value. Just get simple designs and any archaeological evidence will be too corroded to look out of place.
Mink oil made of mink oil and lanolin is used to waterproof leather. Waterproofing would always have been valuable at the time and both ingredients were available and used. They were just a pain in the ass to get in any quantity. Heck, you could just buy a tub of lanolin at any fabric store - years worth of work in one tub!
As Zwol mentioned many goods had monopolies or defacto monopolies (guilds, charters, patents, ect) in certain countries so you could get your friends in serious trouble, so far as I know none of these had monopolies at the time although some did develop ones several centuries later. This is also why I left things like purple dye off the list they were far more restricted and not something you want your friends to be caught with.
For more ideas, I recommend the video series "worst jobs in history" to get an idea of what was valuable at the time.
5I think this might be rather opinion based. – AndreiROM – 2017-03-23T14:38:41.413
3@andreirom rather, but falls into the "good subjective" genre. Of course there will be opinonated answers, but the good ones will be backed by historical facts. – Mindwin – 2017-03-23T14:48:03.843
32@KWeiss: I'd add another requirement: does not get you burned at the stake for dealing with the devil. – Mindwin – 2017-03-23T14:48:34.300
21@mindwin - *the good ones will be backed by historical facts* <- none of the answers so far as backed by much of anything. – AndreiROM – 2017-03-23T14:49:19.423
15@andreirom then downvote the crappy answers. The question has nothing to do with it. – Mindwin – 2017-03-23T14:50:23.510
22They will be impressed enough that you have all your teeth. – user535733 – 2017-03-23T14:52:22.210
1@AndreiROM: I provided links to back up every single one of my suggestions. – TheBlackCat – 2017-03-23T15:00:15.863
2Are you stuck in the past? If not, do you plan on going back and forth to resupply? What kinds of things are you going to be bribing the people to do? The bribe will be one thing if you just want some free lodging and food as you wander the countryside and explore the past. It will be completely different if you want the people to fight for you as you carve out your own pocket empire. – Shane – 2017-03-23T15:48:06.497
You don't say what you want to get back for your trades, @KWeiss. Gold? Swans? Marriage? In that era and place, gold was a currency with a well–determined value in trades, but it was hardly a monetary standard. – can-ned_food – 2017-03-24T03:26:18.907
1Mildly related: the "Magic 2.0" series by Scott Meyer touch on some of this. There, though, the premise is the entire world is a computer simulation which certain people can "hack into" to do (almost) anything they want, including time-travel. Impressing natives is mostly through reality-hacking special effects and conjuring tricks. (No affiliation: just liked the books). – TripeHound – 2017-03-24T11:57:00.840
1seems to me that the 'dont leave evidence' criteria is too hard. Virtually anything could survive – Ewan – 2017-03-25T00:54:35.727
1Ideally it wouldn't leave evidence. You can never be 100% sure it won't, but e.g. some spices would be much less conspicuous than batteries or plastic items. "What could I bring on my time travel" questions are common on this site, if this one is more opinion based than the others please suggest a way to improve it. Thank you for your comments! – user2727 – 2017-03-27T09:56:49.450
3so let me get this straight... you have a time machine, but.... no money. a time machine, and no money? let's just think about this premise for a bit. – Michael – 2017-03-27T17:44:25.903
1Literally money. Pennies and nickles look like valuable money, but are really only nickles to you. – 458 – 2017-03-27T17:48:49.790
Is weight or size of the items a concern? – Brian Risk – 2017-03-27T18:03:26.317
2
It might be time to revisit https://worldbuilding.meta.stackexchange.com/questions/2381/definition-of-when-a-question-is-about-character-action and related discussions (e.g, but see also links/related), as questions like this seem to be popping up a lot recently, and by all accounts in all related meta discussions so far, are categorically off-topic, noting that this is about the actions of a specific character (the time traveller). If these questions are to become acceptable the policy should be reevaluated, otherwise the questions should be more closely monitored and the policies upheld.
– Jason C – 2017-03-27T19:26:40.647Check the basic human needs, first.
I would say: contraceptive pills.
You can promise a prince or a king to bang whomever he wants, with full pleasure (except STD) without any consequence.
Silphium may have been a contraceptive, and may have come extinct because of an exceptional demand. Contraceptive pills became a step of "women's freedom to do whatever they want with their body" (you know what it means).
Well, you can also get burned by church officials by promoting fornication.
Just remember: sex sells. – Tuan Trinh – 2017-03-28T15:29:39.980
This is really broad. There are a lot of people alive in the middle ages and they all have different wants and needs. – James – 2017-04-17T19:37:39.320